Stepper motors are motors whose normal operation consists of discrete rotational motions, or steps, of the output shaft. The physical arrangement and number of stator teeth and rotor teeth determine the number of degrees of output shaft rotation per step. Each rotational step is of essentially uniform magnitude, i.e. the same number of arc degrees, rather than the continuous spinning that is characteristic of a continuous duty motor. Properly driven, a stepper motor can also operate at different velocities and with different torque outputs. And, since they index in fixed annular increments rather than continuous motion, a stepper motor may be advanced to a particular position and magnetically latched at the precise point to which it is stepped. Thus, stepper motors may act as a transductor to convert digital electric signals from a microprocessor or digital computer to mechanical motion.
Each digital input signal that excites the stepper motor typically causes it to increment through one precise step or angle of motion. A controller for a stepper motor generates at the appropriate time the digital excitation pulses of a predetermined frequency, duration and magnitude to match the desired output. These pulses are applied to and excite each stepper motor winding in turn with a certain energy during the driving phase to obtain the desired discrete starting, rotation and stopping motion. Typically, a voltage pulse is applied to the motor winding for a predetermined time to drive current through the winding. The resulting magnetomotive forces cause the rotor to rotate, and the motor generates a counter EMF as the shaft begins to spin.
A stepper motor winding is a typical inductive load which appears as a series connected inductance and resistance. Thus, when a voltage is applied to the motor winding, the current rises exponentially with the time constant of L/R (inductance/resistance) seconds. Conventional wisdom teaches that the winding current should rise as quickly as possible when the motor winding is first energized during a driving phase. This purportedly increases and improves the speed, power and torque output characteristics of the motor. And, since the current rise for a typical inductive load is directly proportional to the applied voltage, a higher than rated voltage is applied to a winding when it is first energized during each driving phase or step. However, the steady state current must be maintained below a certain level to prevent damage to the motor, so the voltage is typically reduced after a certain period of time.
The prior art discloses numerous dual voltage drives which use a high voltage to build current in the windings and then switch to a lower voltage to either decrease current flow or maintain a minimum current flow. This reduces circuit power dissipation and improves performance. Such a drive is often used in "move and hold" applications where a fast rise to a high current is necessary to rotate the shaft, but a small current is sufficient to hold it in place. Due to the increasing complexity and sophistication of stepper motor drive circuits, and the extreme cost and hardship associated with wholesale replacement of hardware already in place, it is desirable to improve stepper motor performance with minimal changes to existing drive circuits.
It is an object of the present invention to provide an improved method and apparatus for driving a stepper motor with multiple voltages.
It is a further object of the present invention to provide a method and apparatus for driving a stepper motor that increases the torque output without increasing the voltages applied to the stepper motor windings. This allows stable motor operation using lower voltage power supplies.
It is a still further object of the present invention to provide a method and apparatus for driving a stepper motor with multiple excitations that provide improved power output by changing the timing of the excitation of the motor windings.
These and other objects are accomplished by the present invention which drives the stepper motor by applying a first excitation to a motor winding and sequentially applying a second excitation having a voltage higher than the first excitation to the same winding during the same driving phase. The second excitation is applied at least just prior to and continuing substantially until the time that the winding is de-energized at the end of the driving phase.